Fig. 1

Preoperative imaging results (a, b). The radioparent lesion in the patella can be seen in the anteroposterior and lateral X-rays (c, d). The focus with a sharp edge, in a lobulated or worm-eaten shape, was observed in the axial and coronal views of preoperative CT. No boundary or internal calcification was noted in the focus (e). At the sagittal view of preoperative CT, the focus was connected with the joint cavity at the junction between the inferior patella and the articular surface (f-h). In preoperative MRI, f was T1WI, the patella focus was hypointensity, and g and h were T2WI. The focus was hyperintense and contained the fluid level. Therefore, secondary ABC was suggestive